1. Field of the Invention
The present invention relates generally to a class of field effect transistors based on III-nitride materials, and relates more particularly to a field effect transistor layout geometry for carrying high current.
2. Description of Related Art
Column III-nitride alloy used in semiconductors, including AlN, GaN and InN may be used to produced devices that exhibit large dielectric breakdown fields, and are capable of high power, high frequency switching. A typical application for III-nitride semiconductors is in the construction of emitters for cell phone base stations. III-nitride semiconductors have a large dielectric breakdown field of greater than 2.2 MV/cm, while providing high mobility for carriers, making them excellent for power applications.
The device is fabricated for the high power applications where III-nitride materials are advantageous, have a high electron mobility and are referred to variously as heterojunction field effect transistors (HFETs), high electron mobility transistors (HEMTs) or modulation doped field effect transistors (MODFETs). These types of devices are typically able to withstand high voltages such as in the range of 100 Volts, while operating at high frequencies, typically in the range of 2–100 GHz. These types of devices may be modified for a number of types of applications, but typically operate through the use of piezoelectric polarization fields to generate a two dimensional electron gas (2DEG) that allows transport of very high current densities with very low resistive losses. The 2DEG is formed, for example, at an interface of AlGaN and GaN materials in these conventional III-nitride HEMT devices.
The formation of a high electron mobility channel in III-nitride semiconductor materials permits devices to be constructed that are capable of operating well in high current applications. This feature of III-nitride semiconductor devices results in an improvement by a factor of ten in the resistance area product (RA) when compared to state of the art silicon based devices. The switching speed of these devices is also many times higher than that of conventional silicon devices, while switching losses are much lower. These attributes all contribute to permitting the construction of high frequency, low loss devices for high current applications.
Previously, III-nitride semiconductor devices fabricated for high voltage, high frequency applications have often been formed as planar devices with ohmic contacts located on either side of a gate contact that controls the conduction channel between the two ohmic contacts. This type of planar structure is illustrated in FIG. 1 in device 10. Device 10 is a simple MISFET constructed using III-nitride materials. Device 10 includes a body layer 15, overlaid by a barrier layer 17, both of which are composed of III-nitride materials. Layer 17 has an in-plane lattice constant that is lower than that of layer 15, which contributes to producing interface strain and high electron mobility 2DEG 13. A source 12 and a drain 14 are coupled to barrier layer 17 through low resistance ohmic contacts 18, to form current carrying electrodes for device 10. A gate 16 is positioned between source 12 and drain 14, and is separated from layer 17 by a dielectric layer 11. The application of an electric potential to gate 16 interrupts the conduction channel formed by 2DEG 13, to prevent current conduction between source 12 and drain 14. Device 10 is accordingly a depletion mode, or nominally on device. Exemplary constructs of device 10 provide for body layer 15 to be composed of GaN, while area layer 17 is composed of AlGaN. Source 12 and drain 14 offer the same functionality, and may be interchangeable, or simply referred to as ohmic contacts. Gate 16 is formed as an electrode and is composed of a conductor, such as metal or conductive semiconductor material.
Device 10 illustrates a III-nitride semiconductor structure that is capable of switching high voltages dependent upon the thickness of dielectric layer 11, and the separation of gate 16 from either of source 12 or drain 14. A MISFET constructed according to the structure of device 10 may carry up to approximately 1 A/MM per unit of gate length on the device. These types of devices have been used successfully in low current, high power, high frequency applications. For high current applications, such as applications that may control ten or more amps of current, the structure of device 10 is impractical due to the dimensional requirements of a device in such an application. For example, if device 10 were constructed for use with an application in which ten or more amps of current were switched and controlled, device 10 would have dimensions of approximately 10 microns by 10 mm, which would not be practically suitable for use in real world applications. Accordingly, it would be desirable to produce an HFET device such as a MISFET that is capable of controlling ten or more amps of current, while meeting the dimensional needs of real world applications.
Designs to overcome the above drawbacks have been previously presented in silicon devices, where the resultant device is much more compact and is manufactured and handled with greater ease for real world applications. A plan view of a high current MISFET device 20 is illustrated in FIG. 2. Device 20 has interdigitated portions of a drain electrode 22 and a source electrode 24 extending between an alternating with each other. A gate electrode 26 is formed in a serpentine shape between interdigitated drain portions 23 and interdigitated source portions 25. Gate electrode 26 is separated from drain and source electrodes 22, 24 with a dielectric 21. Device 20 may be a smaller segment of a larger device, where power connections are made to device 20 along runners 22A and 24A to contact drain and source electrodes 22, 24, respectively. A number of devices 20 may be ganged together to obtain a desired current rating, while individual devices 20 are constructed to have desired maximum voltage ratings.
Device 20 provides a more symmetrical design widely used in presently fabricated current control devices. However, when a III-nitride semiconductor device is constructed according to the configuration of device 20, the higher current applications for which such a device is used introduce additional problems in the operation of such a device. Due to the large amounts of current passed between drain and source electrodes 22, 24, under gate electrode 26, the resistance of the material used to construct electrodes 22, 24 increases in significance with respect to overall operation of device 20. Accordingly, current flowing through runners 22A, 24A tends to diminish towards the tips of finger portions 23, 25, due to the resistance of the material used to make finger portions 23, 25. More current flows in finger portions 23, 25 closer to runners 22A, 24A, respectively than at the tips of finger portions 23, 25. This variation in current flow due to the resistance of finger portions 23, 25 results in a power loss related to the length of finger portions 23, 25. Accordingly, it would be desirable to produce a design for a MISFET structure using III-nitride semiconductor material that has a lower on resistance, with improved resistance-area product (RA).
A drawback of III-nitride HEMT devices that permit high current densities with low resistive losses is the limited thickness that can be achieved in the strained AlGaN/GaN system. The difference in the lattice structures of these types of materials produces a strain that can result in dislocation of films grown to produce the different layers. This results in high levels of leakage through a barrier layer, for example. Some previous designs have focused on reducing the in-plane lattice constant of the AlGaN layer to near where the point of relaxation occurs to reduce the dislocation generation and leakage. However, the problem of limited thickness is not addressed by these designs.
Another solution is to add insulation layers to prevent leakage problems. The addition of an insulator layer can reduce the leakage through the barrier, and typical layers used for this purpose are silicon oxide, silicon nitride, saphire, or other insulators, disposed between the AlGaN and metal gate layers. This type of device is often referred to as a MISHFET and has some advantages over the traditional devices that do not have an insulator layer.
While additional insulator layers can permit thicker strained AlGaN/GaN systems to be constructed, the confinement layer produced by the additional insulator results in lower current carrying capacity due to the scattering effect produced on electrons at the GaN/insulator interface. Also, the additional interface between the AlGaN layer and the insulator results in the production of interface trap states that slow the response of the device. The additional thickness of the oxide, plus the additional interfaces between the two layers, also results in the use of larger gate drive voltages to switch the device.
Conventional device designs using nitride material to obtain nominally off devices rely on this additional insulator to act as a confinement layer, and may reduce or eliminate the top AlGaN layer. These devices, however, typically have lower current carrying capacity due to scattering at the GaN/insulator interface.
Accordingly, it would be desirable to produce a nominally off HEMT switching device or FET that has a low leakage characteristic with fewer interfaces and layers that can still withstand high voltage and produce high current densities with low resistive losses. Presently, planar devices have been fabricated with GaN and AlGaN alloys through a number of techniques, including MOCVD (metal organic chemical vapor deposition) as well as molecular beam epitaxy (MBE) and hydride vapor phase epitaxy (HVPE).
Materials in the gallium nitride material system may include gallium nitride (GaN) and its alloys such as aluminum gallium nitride (AlGaN), indium gallium nitride (InGaN) and indium aluminum gallium nitride (InAlGaN). These materials are semiconductor compounds that have a relatively wide direct bandgap that permits highly energetic electronic transitions to occur. Gallium nitride materials have been formed on a number of different substrates including silicon carbide (SiC), saphire and silicon. Silicon substrates are readily available and relatively inexpensive, and silicon processing technology has been well developed.